Python property descriptor design: why copy rather than mutate?

Question

I was looking at how Python implements the property descriptor internally. According to the docs property() is implemented in terms of the descriptor protocol, repr

Answer 1

So you can use properties with inheritance?

Just an attempt at answering by giving an example:

class Base(object):
    def __init__(self):
        self._value = 0

    @property
    def value(self):
        return self._value

    @value.setter
    def value(self, val):
        self._value = val


class Child(Base):
    def __init__(self):
        super().__init__()
        self._double = 0

    @Base.value.setter
    def value(self, val):
        Base.value.fset(self, val)
        self._double = val * 2

If it was implemented the way you write it, then the Base.value.setter would also set the double, which is not wanted. We want a brand new setter, not to modify the base one.

EDIT: as pointed out by @wim, in this particular case, not only it would modify the base setter, but we would also end up with a recursion error. Indeed the child setter would call the base one, which would be modified to call itself with Base.value.fset in an endless recursion.